1. Field of the Invention
The present invention relates to an image processing apparatus, a method for reshaping a threshold matrix, and a computer-readable medium.
2. Description of Related Art
A screen process determines whether to put a dot at a pixel of image data and determines the size of the dot using a threshold matrix having thresholds defined for the respective pixels. The threshold matrix defines halftone dots each consisting of a group of dots, and the size and density of the halftone dot define a gradation.
Examples of well-known conventional methods for forming a threshold matrix for frequency modulation (FM) screening include the simulated-annealing method, the void-and-cluster method, and the binary pattern pair correlation construction algorithm (BIPPCCA) (See U.S. Pat. No. 6,798,537 and Daniel L. Lau and Gonzalo R. Arce, Modern Digital Half toning, second edition, U.S.A.: CRC Press).
All the aforementioned techniques determine a threshold for each pixel based on an initial pattern of randomly arranged dots by random numbers.
The granularity of an image after the screen process is improved as the shape of each halftone dot formed with the threshold matrix is closer to a circle.
Unfortunately, the threshold matrix formed with a conventional method causes unstable shapes of halftone dots. With reference to FIG. 9, many of the halftone dots have an irregular and uneven shape. Moreover, an isolated dot arises among the halftone dots. Consequently, the screen process using such a threshold matrix produces a grainy image. This is partly because the threshold matrix is created from an initial pattern formed with the random numbers.